[RFR] 8196969: JTreg Failure: serviceability/sa/ClhsdbJstack.java causes NPE

[Chris Plummer]

Hi Severin,

Sorry, this is not an area that I have any expertise in. However, I did 
confirm that it fixes the NPE I was seeing with JShellHeapDumpTest.java, 
which brings up a question. You said this happens with -Xcomp, but I was 
never using -Xcomp. Might it also be triggered without -Xcomp?

thanks,

Chris

On 10/1/19 1:58 AM, Severin Gehwolf wrote:
> Anyone? Chris maybe?
>
> On Fri, 2019-09-27 at 14:12 +0200, Severin Gehwolf wrote:
>> Hi,
>>
>> Could I please get reviews for this SA fix? The issue only happens
>> intermittently and with -Xcomp. The new regression test reproduces the
>> issue somewhat reliably. I got 10/10 fails for unpatched, but I've seen
>> it pass as well.
>>
>> When the issue happens, PCDesc's getScopeDecodeOffset() returs 0
>> (DebugInformationRecorder.SERIALIZED_NULL). The current SA code doesn't
>> handle this case and goes on and tries to read ScopeDesc from the
>> DebugInfoReadStream at the bogus offset. From then on, bad things
>> happen. A NPE in StackTrace could be one symptom.
>>
>> The same code in hotspot deals with serialized null differently. It
>> doesn't read from the debug info stream, and manually sets up a
>> reasonable frame. Note decode_body is called from ScopeDesc's
>> constructor where decode_offset might have been set to 0:
>>
>> void ScopeDesc::decode_body() {
>>    if (decode_offset() == DebugInformationRecorder::serialized_null) {
>>      // This is a sentinel record, which is only relevant to
>>      // approximate queries.  Decode a reasonable frame.
>>      _sender_decode_offset = DebugInformationRecorder::serialized_null;
>>      _method = _code->method();
>>      _bci = InvocationEntryBci;
>>      _locals_decode_offset = DebugInformationRecorder::serialized_null;
>>      _expressions_decode_offset = DebugInformationRecorder::serialized_null;
>>      _monitors_decode_offset = DebugInformationRecorder::serialized_null;
>>    } else {
>>      // decode header
>>      DebugInfoReadStream* stream  = stream_at(decode_offset());
>>
>>      _sender_decode_offset = stream->read_int();
>>      _method = stream->read_method();
>>      _bci    = stream->read_bci();
>>
>>      // decode offsets for body and sender
>>      _locals_decode_offset      = stream->read_int();
>>      _expressions_decode_offset = stream->read_int();
>>      _monitors_decode_offset    = stream->read_int();
>>    }
>> }
>>
>> The proposed patch handles serialized null scopes similar to the
>> hotspot side of things, by returning a null scope. CompiledVFrame
>> already deals with null scopes when in debugging mode.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8196969
>> webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8196969/03/webrev/
>>
>> Testing: tier 1 tests on Linux x86_64 (release/fastdebug). jdk-submit
>> and ran various reproducer tests including 1000 interations of the
>> added regression test. All pass.
>>
>> Thoughts?
>>
>> Thanks,
>> Severin